atoms and molecules


Main picture: Maria Rodas Verde aligns the quantum optical chip. Inset: Pete Shadbolt.


spookY ActIon


Physical laws place natural constraints on what is possible, and what is not. It is a remarkable fact that these same laws also restrict the scope of operations that can be performed with information, says Pete Shadbolt.


WHETHER HEWN ON STONE tablets, stored on a memory stick, or encoded in the DNA of a pygmy shrew, information is bound to real physical systems and laws. It is therefore possible to construct information processing tasks which can be solved efficiently within one physical regime, but are effectively impossible – demanding more resources than exist in the known universe, or a computer larger than the sun – in another. Quantum mechanics is the set of rules which


best describe nature at the scale of single atoms and molecules. It is notorious for its counter- intuitive strangeness and “spooky” (Einstein’s quote) behaviour, phenomena which are very far removed from everyday experience and are not accounted for by classical physics. By encoding information in the state of nanoscale objects such as atoms, electrons and photons, these new rules and shortcuts can be exploited for information processing tasks. In principle, this would allow a quantum mechanical machine – a quantum computer – to solve some ‘classically impossible’ problems, such as simulating new chemicals, as well as tackling deep questions in computer science and mathematics. Researchers from the University of Bristol’s Centre for Quantum Photonics (CQP) are now


working to build such machines, encoding, manipulating and measuring information using the quantum state of single photons – particles of light. Teir latest development is a tiny chip, which can be reconfigured to perform several experiments that would each ordinarily be carried out on a bench the size of a dining table. Te chip, which measures 70 millimetres


by three millimetres, is made from silica (glass) using commercial semiconductor fabrication techniques, and uses two photons to encode quantum information and generate entanglement, the fundamental resource that gives quantum computers their power. Te researchers are now working on scaling up the complexity of such devices, using a greater number of photons and more complicated circuits. “In order to build a quantum computer, we


need not only to be able to control complex phenomena such as entanglement and mixture, but we need to be able to do this on a chip, so that we can scalably duplicate many such miniature circuits – in much the same way as the modern computers we have today,” says Professor Jeremy O’Brien, Director of the CQP. “Our device enables this, and we believe it is a major step forward towards optical quantum computing.”


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GLOSSARY Photon: A single particle of light. The sun emits about 400,000, 000,000,000,000,000,000, 000,000,000,000,000,000, 000 photons every second.


Quantum photonics: Historically, experiments in optical quantum computing have been performed using ‘bulk optics’, where sugar- cube sized chunks of glass and crystal are bolted to large benches. Quantum photonics shrinks these circuits onto chips.


Entanglement: Where quantum particles are intrinsically linked, despite being separated by large distances. Entanglement is the basic resource which gives quantum computers their power.


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